A diagnostic method of diagnosing a type of a field device, includes: setting a plurality of connection units included in an I/O module to be in a state of being capable of inputting or outputting a hybrid signal which is an analog signal having a digital signal superimposed thereon, the connection units being connected to the field device and capable of inputting the hybrid signal, outputting the hybrid signal, inputting a digital signal, and outputting a digital signal; and diagnosing the type of the field device which is connected to the connection units of the I/O module, based on the digital signal included in the hybrid signal obtained through the connection units.